This application is the national phase of international application PCT/GB99/00841 filed Mar. 18, 1999 which designated the U.S.
This invention relates to a composition for impregnation of a lignocellulosic material, and to a method of preparing a finished product from a lignocellulosic material which has been so impregnated.
It is well known to preserve timber by impregnating preservatives in a pressure cylinder, usually with a vacuum/pressure, vacuum/vacuum, pressure/pressure or pressure/pressure/vacuum cycle. Examples of suitable preservatives are the copper chrome arsenates in water, creosotes, and MDIs with or without anhydrides in non aqueous solvents.
Desirable properties to be imposed upon wood are resistance to water ingress, resistance to the movement of water in the material by capillarity, preventing attack of the lignocellulosic material by micro organisms, particularly fungi or insects such as termites, stabilising the wood dimensionally, improving its mechanical properties and possibly also improving its appearance. Very few of the known impregnation technologies accomplish all of these objectives.
According to a first aspect of the invention, there is provided a composition for impregnation of a lignocellulosic material, which composition comprises:
(a) a high boiling point tar acid or a pitch derived from coal;
(b) a solvent for component (a) selected from the group consisting of a low carbon alcohol, an azeotrope of a low carbon alcohol and another solvent, and a mixture of a lower carbon alcohol and water;
(c) a phenol formaldehyde resole resin;
(d) optionally an acid catalyst for the phenol formaldehyde resole resin; and
(e) optionally a formaldehyde donor or a precursor of formaldehyde.
By a lignocellulosic material there is meant any plant material emanating from the photosynthetic phenomenon.
The lignocellulosic material may, for example, be chosen from wood in sawn, sliced, peeled or veneer form; plywood; paper; woven strands; fibres, particles, flakes or chips of wood or agricultural products; or the like.
A preferred impregnating composition of the invention comprises:
(a) a high boiling point tar acid or a pitch derived from coal in an amount of from 2% to 60% inclusive by mass, more preferably from 8% to 15% inclusive by mass of the combined mass of components (a), (b) and (c);
(b) a solvent selected from the group consisting of a low carbon alcohol, e.g. methanol, ethanol, propanol or butanol, preferably methanol; an azeotrope of a low carbon alcohol and another solvent, and a mixture of a low carbon alcohol and water; in an amount of from 50% to 97% inclusive by mass, more preferably from 77% to 90% inclusive by mass of the combined mass of components (a), (b) and (c);
(c) a phenol formaldehyde resole resin in an amount of from 1% to 30% inclusive by mass, more preferably from 2% to 8% inclusive by mass of the combined mass of components (a), (b) and (c);
(d) an acid catalyst for the phenol formaldehyde resole resin in an amount of from 1% to 15% inclusive by mass, more preferably from 4% to 6% inclusive by mass of the resin; and
(e) a formaldehyde donor or a precursor of formaldehyde, such as hexamethylene tetramine, in an amount of from 1% to 15% inclusive by mass, more preferably from 8% to 12% inclusive by mass of the mass of the high boiling point tar acid or the pitch.
The solvent, component (b), also acts as an extending liquid for the phenol formaldehyde resole resin.
The impregnating composition may also contain other components such as an antioxidant and a deodoriser, both of which should be miscible or soluble in the extending liquid.
A preferred level of impregnation for upgrading wood for exterior exposure is, for example, 2% by mass of resin with 10% by mass of pitch on the mass of the dry wood.
According to a second aspect of the invention, there is provided a method of preparing a finished product from a lignocellulosic material, including the steps of:
(i) impregnating the lignocellulosic material with an impregnating composition as set out above;
(ii) removing the solvent from the product of step (i), optionally with recovery thereof for reuse; and
(iii) subjecting the product of step (ii) to suitable conditions of temperature and optionally of pressure to polymerise or to set the resin, to form the finished product.
Step (i) of the method of the invention may be carried out by immersing the lignocellulosic material in the impregnating composition, or by impregnating the lignocellulosic material with the impregnating composition under suitable conditions of vacuum and pressure.
In step (ii) of the method of the invention, the solvent may be removed from the product of step (i) in any suitable way.
In step (iii) of the method of the invention, the product of step (ii) may be subjected simply to an elevated temperature, for example, a temperature exceeding 90xc2x0 C., to allow the resin to polymerise or set. Alternatively, when the lignocellulosic material is in particulate form, the product of step (ii) may be subjected to suitable conditions of temperature, e.g. a temperature exceeding 90xc2x0 C., and pressure, for example, between the platens of a press, to allow the resin to polymerise or set and thus to adhere the particles to one another to form the finished product.
According to a third aspect of the invention, there is provided a finished product comprising a lignocellulosic material which has been impregnated with an impregnating composition as set out above, the impregnated lignocellulosic material having been subjected to suitable conditions of temperature and optionally of pressure to polymerise or to set the resin, to form the finished product.